The Universal Serial Bus (“USB”) was designed to provide a serial communication channel between computers and peripheral devices. For example, USB can connect computer peripherals such as such as mice, keyboards, gamepads, joysticks, scanners, external drives, etc. to a computer. While USB was designed for personal computers it has become commonplace on battery powered computerized devices such as PDAs, music players and cellular telephones which use USB for both data communication and to recharge their batteries. The design of USB is standardized by the USB Implementers Forum (USBIF), an industry standards body incorporating leading companies from the computer and electronics industries.
There are several types of USB connectors approved by the USBIF, including those with four contacts (pins or sockets), such as the USB-A and the USB-B connectors, as well as those with five contacts (pins or sockets), such as the mini/micro-A, mini-micro-B and mini/micro AB. Most computers, including laptop computers, have several USB-A connectors, each of which has a power (VBUS) contact, ground (GND) contact and two data line contacts (D+ and D−).
Laptop computers are becoming increasingly popular. In order to preserve battery life, most laptop computers have “inactive” modes where they are not fully on or fully off, such as “sleep”, “standby” and certain “hibernate” modes. During operation, such computers are considered to be in their “active” state, and their batteries may last for a number of hours. However, by limiting current draws, the batteries of computers in an “inactive” state can last for days.
With some exceptions, laptop computers can charge compliant USB devices that are plugged into a USB port of the computer when the computers are in an active state. In such cases, the laptop computer is considered to be a “USB host.” The devices that can be charged through the USB include, but are not limited to, cellular telephones, music players, PDAs etc., collectively referred to herein as “USB devices.” The ability to charge USB devices through the same USB port used for the transfer of data is very convenient and is becoming increasingly popular.
It should be noted that USB devices that do not conform to accepted standards (“non-compliant USB devices”) can always draw current from a USB connector that has power on its VBUS contact. However, there is a strong and increasing desire for USB devices to be compliant with USB standards. For example, USBIF rules specify that a USB device (one type of “compliant USB device”) can only draw current from a computer when the computer is in an active mode and gives its permission. For example, some laptops will not allow charging through a USB connector if it is running solely on battery power. This means that if a laptop computer is in an inactive mode the USB device cannot be charged through the laptop's USB connector because it cannot communicate with the compliant USB device. Instead, the USB device can be charged by a dedicated USB charger (“dedicated charger”) which is essentially a power adapter with an AC input and a USB connector output. The dedicated charger has an identification protocol which lets a USB device know that it is connected to dedicated charger.
There are several dedicated charger identification (“ID”) protocols currently being used. One, implemented by Apple Computer, Inc. of Cupertino, Calif. (“Apple”), uses resistive voltage dividers coupled to the D+/D− contacts of the USB connector as illustrated in FIG. 1. More particularly, the circuit inside of an Apple dedicated charger includes a pair of resistive voltage divider circuits, a first of which couples the series connection of a 75 K′Ω resistor and a 49.9 K′Ω resistor between a 5.0 volt voltage source and ground, and a second of which couples the series connection of a 43.2 K′Ω resistor and 49.9 K′Ω resistor between a 5.0 volt voltage source and ground. The center nodes of the two voltage dividers are coupled to the D+ and D− contacts, respectively, of the USB connector. An Apple iPod® or iPhone® USB device (another type of “compliant USB device”), uses a voltage detector to detect the voltages on the D+ and D− contacts as an identification protocol for an Apple dedicated charger.
Another dedicated charger identification protocol is specified by the USBIF. With this protocol, the D+ and D− contacts are shorted as seen in FIG. 2. There is a circuit inside of a compliant USB device as illustrated in FIG. 3 which can detect the short between the D+ and D− contacts to verify that it is connected to a dedicated charger. China has also adopted this convention on a national basis for USB dedicated chargers.
There are other proprietary dedicated charger ID protocols. For example, Motorola uses 5 contact micro and mini USB connectors with its cell phones and has its own proprietary protocols for the identification of dedicated chargers. However, micro and mini USB connectors are not typically provided on laptop computers.
With the USBIF protocol, the circuit inside the USB device detects when the D+ and D− contacts are shorted together. This circuit is illustrated in FIG. 3. When a voltage is detected by the USB device on the USB bus, a voltage is applied to the D+ contact and a load is coupled to the D− contact. Using a window comparator and a debounce timer the circuit determines whether the voltage on the D+ contact is the same as the voltage on the D− contact, identifying whether the D+ and D− contacts are shorted together or not. A description of the current USBIF battery charging specification can be found at www.usb.org/developers/devclass_docs#approved and entitled “Battery Charging Specification, Rev. 1.1, Apr. 15, 2009, incorporated herein by reference.
To address the problem of not being able to charge a compliant USB device on a computer unless it is in an active mode, Fairchild Semiconductor Corporation has proposed a solution as illustrated in FIG. 4. Based upon the limited information available, it is believed that the Fairchild protocol is triggered by the detection of current on the VBUS contact of a USB-A port. Next, it is believed that the device shorts the D+ and D− contacts of the USB-A port to emulate a dedicated charger following the USBIF protocol. However, it is believed that if the current sensed is less than a predetermined threshold level the device determines that the device is an Apple USB device and must reset the Apple USB device detection circuit by cycling VBUS off and then back on again. A DPDT switch is thrown to connect voltage divider resistors to the D+ and D− contacts in conformance with the Apple dedicated charger protocol. If the current sensed with the voltage dividers is greater than the current sensed without the voltage dividers the switch will remain set and the Apple USB device will charge. However, if the current sensed with the voltage dividers is not greater than the current sensed without the voltage dividers, VBUS is again cycled off and on to reset the USB device's detection circuit and the switch is again activated to short the D+ and D− contacts. When the current sensed is zero, the switch is opened and control is reset, with VBUS remaining on to charge the USB device.
While the Fairchild proposal attempts to address the problem of charging Apple and USBIF compliant USB devices from a USB port of an inactive computer, practical implementation details remain significant. First, the VBUS must be monitored. Second, decisions must be made as to current thresholds. Third, VBUS may have to be repeatedly turned off and on as the device iterates through the different possible modes. The circuitry and algorithms of the Fairchild proposal are therefore complex.
These and other limitations of the prior art will become apparent to those of skill in the art upon a reading of the following descriptions and a study of the several figures of the drawing.